Pickling of non-ferrous metals

1. Characteristics and properties

The pickling of copper and its alloys serves both to achieve certain surface qualities and to clean the workpieces before electroplating. It takes place in concentrated acids, which lead to etching of the scale layer as well as the oxides and sulphides. At the same time, the surface is leveled. The removal of the uppermost workpiece surface depends on the pickling agent used, the temperature and the exposure time. This effect can be used to pickle workpieces true to size or to eliminate slight degree or chip formation.

In the case of very heavily scaled surfaces, the material removal can amount to several µm depending on the base material. Pickling is based on a purely chemical reaction and leads to material attack on all surfaces of the workpiece, i.e. also in drilled holes. The pickling of non-ferrous metals with nitric acid mixtures is also known as firing or yellow firing.

2. Benefits and areas of application

Pickling is decisive for the later surface characteristics. It is possible to stain the surface with a glossy finish, to give it a matt appearance or just to descale and clean it. Staining is ideal for fashion accessories such as buttons and buckles as well as for gas, water and sanitary applications.

Stained surfaces tend to oxidise again after some time. In order to effectively prevent this, subsequent electroplating, chromating or passivation is recommended.

3. Basic material

All workpieces made of copper and its alloys can be pickled.

Especially in the case of heat-treated workpieces and workpieces produced by casting, there are scale layers and oxidations on the surface based on copper oxides and copper sulphides which can be removed by pickling.

4. Required delivery condition

In order to achieve an optically and technically excellent result through pickling, it is necessary to process workpieces that have a surface quality that permits this. The workpieces must be free of dirt, oils and greases. Silicone-containing, alkali-/alkaline-earth-containing oils and high-pressure additives must be avoided at all costs.

5. Pre-treatment

Before pickling, the workpieces must be thoroughly degreased and cleaned. There must be no oil or grease on the surface, as this would not allow pickling in the areas wetted with oil and would therefore not achieve a perfect result.